首楞嚴義疏注經 6.286

6.286

法身德也。聲於聞中自有動靜，說為有無，非謂聞性是有無也。世人若以不聞聲時號無聞者，聞性已滅，聲塵更起，遣誰更聞？是知聲有，聞性不生；聲無，聞性不滅。生滅既而遍離，由是得名常真實也。

This is the virtue of the dharmakāya. Sound within hearing naturally has movement and stillness, described as presence and absence, but this does not mean that the nature of hearing is present or absent. If worldly people call it 'not hearing' when they do not hear sound, then if the nature of hearing had already ceased, who would be sent to hear again when the dust of sound arises anew? Thus we know that when sound is present, the nature of hearing does not arise; when sound is absent, the nature of hearing does not cease. Since it is completely free from both arising and ceasing, it is named 'permanent reality'.